The use of nasal Continuous Positive Airway Pressure (CPAP) for the treatment of Obstructive Sleep Apnea (OSA) was taught by Sullivan and described in U.S. Pat. No. 4,944,310, which is entitled “Device for Treating Snoring Sickness”. The treatment generally provides a supply of air to a patient's upper airways at pressures, typically in the range of 4 to 20 cm H2O, which acts to “splint” open the patient's airways. Typically, the CPAP apparatus includes (i) a blower for providing a source of pressurised breathable air to a patient, (ii) a patient interface to be removably worn by the patient, and (iii) an air delivery hose for transferring the pressurised breathable air from the blower to the patient interface. The blower typically includes an electric motor and impeller. One end of the air delivery hose or conduit is connected to the output of the blower and the other end is connected to the patient interface.
Some devices for treating SDB or assisting ventilation provide two pressure levels, one during patient inhalation and a different pressure during patient exhalation. The switching between two pressures may be triggered by a timer, a pressure sensor, a flow sensor, a volume sensor or some combination using techniques well known in the ventilator arts.
An automatically adjusting method and device was first described by Sullivan and Lynch in U.S. Pat. No. 5,245,995, which describes a pressure transducer that, in addition to detecting snoring sounds, can detect other respiratory parameters such as the rate of breathing, inhaled air flow volume and inhaled air flow rate. The device also included a feedback system controlling the output pressure of the air source so as to increase the output pressure in response to the detection of snoring or abnormal breathing patterns, and reduce the pressure in the absence of such patterns. The use of such a device can lead to improved patient comfort since patients receive lower pressures during the portion of their sleep when there are no indications of OSA, but higher pressures when they are needed. Examples of devices operating in this manner are the AutoSet® brand of nasal CPAP devices manufactured by ResMed Limited, Australia.
Other conditions may be treated by nasal ventilation such as Cheyne-Stokes breathing, as described in International Patent Application WO 99/61088. Such devices require very accurate measurement of flow and pressure. OSA is an example of a broader class of disorders generally referred to as sleep disordered breathing (SDB). In this specification, a reference to apparatus for the treatment of OSA is intended to include a reference to apparatus for treating SDB. Nasal CPAP apparatus for treating SDB from a special subgroup within the broader group of mechanical ventilators. Whilst mechanical ventilators are often closed systems with respect to airflow, the blower, conduit and patient interface system used for the treatment of sleep disordered breathing is typically an open system with respect to airflow. That is, the system for treating SDB typically includes a deliberate air leak. A deliberate leak is to be contrasted with unintentional leak. The patient interface in a system for treating SDB typically includes a diffuser which produces a deliberate air leak which, amongst other things, reduces rebreathing of exhaled air. In addition as in most systems, there exists the potential for unintentional leak flow. For example, if the mask is not correctly positioned on the face, or unsuitable for a particular face, there may be leak around the periphery of the face contacting portion of the mask. In some applications of SDB treatment, for example to assist in correctly synchronizing the blower flow with spontaneous patient respiratory effort, it is important to measure accurately the leak, both deliberate and unintentional, from the system.
The “black box” which incorporates the blower, switches, power supply and control circuitry is sometimes termed a “flow generator”. Alternatively, a source of high pressure air may be connected to a controllable valve arrangement to provide air at the required pressure and flow rates. All of these systems may be described as controllable sources of breathable gas.
In most modem devices for treating SDB, especially those providing sophisticated therapies, there is a need for the device to be able to measure accurately the pressure in the patient interface and the flow of air to the patient. One way this can be accomplished is to place flow and pressure sensors directly in the patient interface (such as a mask). Another way this can be accomplished is to place the flow and pressure sensors in the flow generator and have a sense tube connected from the flow generator to the patient interface.
Whilst accurate measurements of, for example, pressure and flow can be made directly at a mask, such an arrangement can be inconvenient from a patient's point of view since it may require additional sensing tubes to be carried from the flow generator to the patient interface. Sense tubes can be difficult to assemble, difficult to clean and may become tangled during use. Alternatively, if the characteristics of the conduit and patient interface are known, it is possible to estimate the desired variables, such as pressure and flow, in the mask using measurements in the flow generator.
Hence there is a need for a way to measure the characteristics of the conduit and patient interface. In this way, the sophisticated apparatus for treating SDB can measure accurately the mask pressure without requiring sense tubes to be connected between the flow generator and the mask.
A large variety of mask systems are today available and each has different characteristics, such as different pressure drop along the conduit and diffuser flow. Furthermore, the characteristics of different samples of a given mask system can vary due to variation during manufacturing. In order that a given flow generator be able to work with a range of mask systems, each mask system must be characterized by the manufacturer for use with the flow generator and the characteristics may be stored in the flow generator, for example, or in some other recordable medium device. In the event that new mask systems are developed, the flow generator may need to be returned to the manufacturer to be tested with the new mask system.
The flow generator then generates a flow and pressure model of this particular hose and mask system and uses these parameters to calculate hose pressure drop, diffuser flow and mask leak as part of its normal operation. The procedure is prompted on the LCD display with checks to make sure the operator is doing the right thing. The characterization procedure takes less than 1 minute.
There is a need for a method and apparatus which enables the characteristics of a wide range of patient interfaces and conduits to be determined without requiring that a flow generator be returned to the manufacturer.